1. Field of the Invention
The present invention relates to a silver halide photographic light-sensitive material and, more particularly, to a silver halide color photographic light-sensitive material having high sensitivity, producing low fog, and having good storage stability.
2. Description of the Related Art
Basic properties required for a photographic silver halide emulsion are high sensitivity, low fogging density, and fine graininess.
In order to increase the sensitivity of an emulsion, (1) to increase the number of photons absorbed by a single grain, (2) to increase the efficiency of converting photoelectrons generated by light absorption into a silver cluster (latent image), and (3) to increase developability for effectively utilizing the obtained latent image, are required. Increasing the size increases the number of photons absorbed by a single grain but degrades graininess. Increasing the development activity is an effective means of increasing the sensitivity. In the case of parallel development such as color development, however, the graininess is generally degraded. In order to increase the sensitivity without degrading graininess, it is most preferable to increase the efficiency of converting photoelectrons into a latent image, i.e., increase a quantum efficiency. In order to increase the quantum efficiency, a low-efficiency process such as recombination and latent image dispersion must be minimized. It is known that a reduction sensitization method of forming a small silver nucleus without development activity inside or on the surface of a silver halide grain is effective to prevent recombination.
The method of reduction sensitization has been studied for a long time. Carroll, Lowe et al., and Fallens et al. disclose that a tin compound, a polyamine compound, and a thiourea dioxide-based compound are effective as a reduction sensitizer in U.S. Pat. Nos. 2,487,850 and 2,512,925 and British Patent 789,823, respectively. Collier compares properties of silver nuclei formed by various reduction sensitization methods in "Photographic Science and Engineering", Vol. 23, P. 113 (1979). She adopted methods of dimethylamineborane, stannous chloride, hydrazine, high-pH ripening, and low-pAg ripening. Reduction sensitization methods are also disclosed in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777, and 3,930,867. Not only selection of a reduction sensitizer but also improvements in a reduction sensitization method are described in JP-B-57-33572 and JP-B-58-1410 ("JP-B-" means examined Japanese patent application). In these disclosures, conventional reduction sensitizers are enumerated, and ascorbic acid is included therein. In these disclosures, however, a compound such as thiourea dioxide is considered to be preferable, and thiourea dioxide, silver ripening, and hydrazine are exemplified. Therefore, preferable properties of an ascorbic acid compound as a reduction sensitizer have not been yet found. A method of using the ascorbic acid compound is disclosed in JP-A-57-179835 ("JP-A" means unexamined published Japanese patent application). Techniques of improving storage stability of a reduction-sensitized emulsion are disclosed in JP-A-57-82831 and JP-A-60-178445, but improvements have not reached a sufficient level.
Regardless of the number of studies as described above, an increase in sensitivity is insufficient as compared with that obtained in hydrogen sensitization in which a light-sensitive material is subjected to a vacuum and then treated with hydrogen gas. This is reported by Moisar et al. in "Journal of Imaging Science", Vol. 29, P. 233 (1985). A demand has also arisen for improvements in storage stability of a light-sensitive material containing a reduction-sensitized emulsion.
As described above, the conventional techniques of reduction sensitization are insufficient to satisfy a recent demand for a photographic light-sensitive material with high sensitivity and high image quality. In addition, an emulsion prepared by these sensitization techniques have poor storage stability.